When drilling through subterranean formations in the exploration for oil and gas, it is common practice to drill larger diameter holes at the surface, and successively smaller diameter holes as the well is drilled deeper. When the desired depth is reached for a given wellbore diameter, a tubular casing is cemented in place. This practice allows for the protection of water tables from drilling and production fluids, improves drilling and production efficiency, and protects the wellbore. It is often desirable to drill a hole larger than the inside diameter of the last casing that was set, at some known depth below the surface. This may be desirable, for example, for setting additional casing below this known depth, which will require drilling an annular well bore diameter sufficient for cementing of the lower casing. This creates a special drilling situation since conventional drill bits of the size needed to generate the desired well bore diameter will not fit inside the casing that has already been set. Tools used for these applications are commonly known as underreamers. Other applications for underreamers include enlarging zones for gravel pack completions or to compensate for plastic flow of salt and shale formations.
Two principal tools are commonly used in the drilling industry to achieve the objective of drilling the well bore diameter larger than the drift diameter of the casing. The first tool used for this purpose is known as the "Bi-center bit." The Bi-center bit is an undersized drill bit with a large eccentric cutting structure located off-center above a smaller pilot drill bit that is centered axially with the drill collars. Fielder discloses such a device in U.S. Pat. No. 5,678,644. The Bi-center bit is sized so that while running it into the hole, the smaller pilot bit will be pushed to one side to allow the tool to pass through the inside of the casing. When the Bi-center bit reaches the bottom of the hole, the pilot bit acts as a centered pivot point for the eccentric cutting structure above it. When drilling, the eccentric cutting structure will then rotate around the pilot bit and generate a larger hole than the inside (or drift diameter) of the casing.
Problems are frequently associated with the use of the Bi-center bit. For example, when drilling a soft formation, the pilot bit will be forced the one side of the hole opposite the larger eccentric cutting structure and the resultant hole drilled will be smaller than required. The offset design of the Bi-center bit results in uneven wear of the cutting structure and lower rates of penetration. Furthermore, the torque generated during the use of Bi-center bits fluctuates and can have a damaging effect on the drill string, and the tool is unreliable in controlling the angle of the hole. An additional limitation is the inability of the Bi-center bit to drill out cement or a casing shoe. Due to this limitation, an extra trip is required to drill out cement or a casing shoe when using Bi-center bits.
The second tool used for the purpose of drilling a section of the well bore diameter larger than the drift diameter of the casing is an underreamer. A typical underreamer includes extendable arms pivotally mounted in a housing on hinge pins for movement between a retracted position and an extended position. The underreamer may be hydraulically or mechanically actuated. While the underreamer is being lowered into the hole, the arms will be in the collapsed or retracted position to permit the tool to pass through the inside diameter of the casing. When the underreamer reaches the depth at which it is desirable to increase the well bore diameter, the arms of the underreamer are hydraulically or mechanically actuated into the extended position.
In the past, most underreamers utilized roller cone type cutters. Weber discloses such a device in U.S. Pat. No. 4,064,951. These devices were limited in their effectiveness in many formations, and unreliable as a result of the numerous moving components and sealing systems required for their construction. Roller cone type cutters require bearing systems. The most reliable roller cone cutters also required a lubrication and sealing system. Furthermore, the component parts were subject to breakage that resulted in costly operations to remove the debris from the bottom of the hole. More recently, attempts have been made to build underreamers which utilize synthetic diamond material. Simpson discloses such a device in U.S. Pat. No. 4,589,504. These underreamers were also prone to breakage of the support arms and cutting elements that resulted in costly operations to remove the debris from the bottom of the hole.
Other underreamer designs have been designed to perform only reaming operations to prevent bit sticking, and are of a type that includes a long conical tapered body attached by splines in a conical shell. Deely discloses such a device in U.S. Pat. No. 3,051,255. These tools were designed to improve wellbore concentricity and ensure that the drill bit does not get stuck. Such devices have suffered from difficulties in their manufacture as related to the design, as well as operational limitations. In particular, underreamers that incorporate long tubular sections which are internally tapered are extremely difficult to manufacture with quality tolerances. The inner surface of the cutting lugs cannot mate uniformly with the length of the conical surface traversing the inner surface of the cutting lugs. Another disadvantage of this design is that the radial forces imparted to the cutting lugs generate resultant forces that remove weight from the bit and urge the tool to disengage and return to the retracted position. The result is an unstable tool that cannot tolerate the shock and vibration associated with simultaneous drilling. Another disadvantage of these tools is that they are severely limited in their total expansion capability and are not capable of enlarging the wellbore by any significant amount.
A primary limitation of past underreamer designs has been the necessity to first drill a pilot hole with a conventional drill bit, then remove the entire drill string, assemble the underreamer onto the drill string, and then begin the underreaming operation. This two step drilling process is slow and costly.
Expandable stabilizers may be used during operations designed to increase the hole diameter. The principals and design solutions known to the industry for the construction of underreamers also apply to the construction of expandable stabilizers. The primary difference is that cutting elements are replaced with wear elements.
It is seen that there is a need for a tool with greater strength and reliability to overcome the disadvantages and limitations commonly associated with conventional bi-center bits, underreamers, and expandable stabilizers of the general configurations described above.